ENTRANCE OF PATHOGENIC BACTERIA 49 



by his name, which had to be fulfilled before the chain of evi- 

 dence in a given case could be regarded as absolutely complete. 



1. The organism must be found constantly present in the 

 tissues of the affected animal, in the same relationship, and in 

 the same kind of disease. 



2. The organism must be capable of being isolated from that 

 animal, and grown in pure culture artificially outside the body. 



3. The organism so isolated and freed from all other organisms 

 and from any trace of the tissues of the original animal, must 

 be capable of producing the disease when inoculated into a 

 second susceptible animal. 



4. The organism must then be found in and be recoverable 

 from the tissues of this second experimental animal. 



The great majority of disease-producing bacteria completely 

 fulfil the demands of these postulates. A few still do not, 

 sometimes for lack of a susceptible animal other than man, 

 sometimes for lack of a suitable artificial culture-medium ; but 

 this list is rapidly diminishing as scientific methods of experi- 

 mentation are being steadily improved. In addition to Koch's 

 postulates, the evidence of other methods, based on our know- 

 ledge of immunity, is now utilised ; for example, the Agglutina- 

 tion Test, in which the bacteria are aggregated or clumped 

 together by the action of the serum of an animal or patient 

 suffering from the specific disease caused by that bacterium, e.g. 

 Typhoid and Mediterranean Fevers, etc. In another such test, 

 the bacteria are paralysed and disintegrated by the corre- 

 sponding serum, as in the case of cholera a process known as 

 Bacteriolysis. Another extremely important test is known as 

 the Deviation or Absorption of Complement a very com- 

 plicated reaction of which mention can only be made here, and 

 the best-known example of which is the now well-known 

 Wassermann Reaction for the diagnosis of Syphilis. (For a 

 description of these and other refinements in modern diagnostic 

 methods, such as the Precipitin, Absorption, and Anapliylactic 

 Reactions, reference must be made to the larger textbooks.) 



Methods of Entrance of Pathogenic Organisms into the 

 Body. As already mentioned, many organisms capable of 

 causing disease are already harboured upon or in the body, 

 and only require the assistance of some damage such as a 

 wound or scratch or burn, or of some cause predispos- 

 ing to the weakening of the tissues, say of the alimentary 

 or respiratory tracts. Organisms already present in the skin 

 or mouth, intestine or elsewhere, may thus become pathogenic 

 when the patient's vitality is lowered. The Pneumococcus may 

 cause a sore throat, a bronchitis, or a pneumonia. Staphylo- 



D 



